The invention relates to water treatment and relates particularly, though not exclusively, to treating polluted water using electroflocculation and/or electrocoagulation reactions.
The ready availability of clean water is a key requirement for health and economic prosperity. Accordingly, the treatment of polluted water assumes a new importance with increasing population and consequent demands on fresh water supplies.
Electrolysis-based water treatment techniques represent one kind of method for treating polluted water. In this respect there are three different types of electrolysis-based water treatment processes:
(i) electroflotation;
(ii) electroflocculation; and
(iii) electrocoagulation.
Electroflotation involves the use of inert electrodes in conjunction with a coagulating agent which has been added to the polluted water to be treated. A voltage is applied to the electrodes, and liberated gas bubbles capture coagulated particles, floating them to the surface. This is similar to a process known as dissolved air flotation (DAF) for treating industrially polluted waterxe2x80x94except that the gas bubbles are supplied by electrolysis of the water and not by compressed air. The process has been commercially available since the mid 1970s, and is generally described by Dr Anselm Kuhn in Electroflotationxe2x80x94the technology and waste water applications, Chemical Processing, IPC Industrial Press Ltd, London.
Electroflocculation uses sacrificial electrodes to generate the coagulating agent, but also uses the bubbles liberated at the electrodes to float the contaminants to the surface. A few electroflocculation processes have been attempted before but they have not been successful for various reasons. These limitations include:
the clogging of the electrodes before the metal has been adequately sacrificed; and
the requirement to purpose build each unit for a particular application because of inability of a single set of electrodes to handle a wide array of water conductivities.
Electrode clogging considerably increases the cost of the treatment process due to the high cost of electrode replacement, typically making the process prohibitively expensive. The latter limitation has meant that it is necessary to tailor existing electroflocculation systems for particular water qualities. This has not been found to be a practical solution for commercial viability.
Electrocoagulation also involves the use of sacrificial electrodes to generate the coagulating agentxe2x80x94usually aluminium or iron ions. Once the water has been treated, it is either filtered, allowed to settle or sent to a gas or air flotation unit to remove the contaminants. Electrocoagulation process offers a number of potential advantages, if they can be realised. However, as with electroflocculation, electrocoagulation similarly suffers from problems of electrode clogging and the need for purpose building each system to the particular water to be cleaned.
The electrocoagulation process has been tried for many years but has not been made to work satisfactorily. For example, U.S. Pat. No. 4,872,959 to Herbst et al describes a tubular system in which the water passes between inner, outer and central electrodes, and is designed to add flocculating ions to settle the pollutants to the bottom of the settling tank into which the water will be passed. It uses iron and aluminium anodes, though suggests other metals as well, for example copper cathode, iron anode. It is not very successful because the electrodes have a tendency to clog. Further, the system is designed for specific conductivities of the water, and controlling the dose of coagulating ions required for a particular reaction is not easy.
U.S. Pat. No. 5,372,690 to Gardner-Clayson et al acknowledges that replacement of electrodes is a big problem and uses metallic, preferably aluminium, steel (alloy) or magnesium as method of overcoming the problem. Uses a layer of metal balls, shot, irregular shaped particles to be consumable, instead of sheet. Current is passed from metal anode to these balls etc., which then pass cations into the water and the process repeats itself. It has the problem that large voltages are required to drive the electric current between the anode and cathode. Also there is no guarantee that the metal particles in this shape are well suited for the passage of an electric current. Also, it does not address the problems of variations in electrical conductivity of the water.
U.S. Pat. No. 5,558,755 to Gardner-Clayson et al is based upon a continuation application of the patent noted directly above. The discussed apparatus includes a fluidised bed of metallic particles through which the medium is flowed and through which an electric current is applied by electrodes for agglomerating contaminants in the medium. In order to allow the electrodes to be non-consumable so that they do not require frequent replacement, the particles are consumable.
In both electrocoagulation and electroflocculation, there has been a tendency to use electrodes of the same material, for example, an aluminium anode and an aluminium cathode. In this case, a significant problem is that the electrodes have clogged because of the build up of an oxide type layer across the surface of the electrodes. Aluminium is deliberately oxidised in a process known as anodising, and it is generally thought that the aluminium went into solution at the anode. As a result, it has been considered difficult to overcome this problem of aluminium going into solution at the anode without forming an oxide layer at the anode.
In essence, existing techniques generally suffer from one or more limitations, such as:
The electrodes continually clog up, long before the metal had been adequately sacrificed into solution. This meant that the metal electrodes must be replaced at a high cost, making the process uneconomic.
Variations in the conductivity of the water meant that the process was often difficult to control.
Variations in the amount of pollutant present meant that installations had to be configured for a particular water quality. This again made commercial viability difficult.
It is an object of the present invention to at least attempt to address one or more of these and other limitations associated with existing techniques.
It has been recognised that aluminium goes into solution at the cathode as well as at the anode, and that the reaction rate at the cathode is around 2.5 to 3 times faster than at the anode. The cathode is also the electrode at which hydrogen gas is liberated, generating hydroxide groups and raising the pH of the water. Aluminium is an amphoteric metal and reacts with alkaline radicals (OHxe2x88x92) as well as with acidic conditions (H+). At the cathode, OHxe2x88x92 reacts with water and aluminium to form an insoluble AL(OH)3, which adheres to the cathode, while at the anode, aluminium only goes into solution without forming an oxide layer.
The invention recognises that a water treatment process can be advantageously provided by using the electroflocculation/electrocoagulation principle, in which two electrodes are used in combination. A voltage is applied across the electrodes, with a positive voltage being applied to a sacrificial anode and a negative voltage applied to an inert cathode. This voltage can be either direct current (DC) or rectified alternating current.
The polarity of the electrodes can be periodically reversed to mitigate surface clogging of the aluminium electrode. Reversing the polarity causes the material attracted to one polarity to be driven off by repelling the material which was attracted. Repulsion can occur in a short time interval compared to that which results in clogging due to electric attraction. Reversing the polarity for small periods of time, between a broad duty cycle range of 0.1% and 30% of the time, reduces the build up of electrically deposited material, thus extending effective electrode lifetime.
Using a predetermined amount of charge as an indication of the degree to which the water is treated allows for greater flexibility in the manner in which the water can be conveniently treated allowing, for example, variations in water conductivity to be readily accommodated. A total charge, in terms of Ampere-hours, passed between the electrodes is found to be an effective measure of degree to which the water has been treated, and can accordingly be used to secure appropriate treatment results.
The invention provides a method of treating water by electrolysis by passing a current between an anode and a cathode. The invention also provides a water treatment unit having a storage tank for holding water in which an anode and cathode are immersed for treating the water by electrolysis. The invention further provides a power supply apparatus suitable for use in assisting to perform the inventive method described above. In one aspect of the invention, the direction of the instantaneous electric current passed between the electrodes is periodically reversed to discourage clogging of the sacrificial electrode, so that the sacrificial electrode acts primarily as an anode and the inert electrode acts predominantly as a cathode. In another aspect of the invention, the instantaneous electric current is delivered so that a predetermined amount of charge is passed through the water, the predetermined amount of charge being indicative of a degree to which the water has been treated.
In preferred embodiments, a series of electrodes can desirably be used to accommodate water of varying conductivity. The number of electrodes used can be externally selectable, so that if the conductivity decreases more electrodes are switched in to increase the area and thus get greater current from the same voltage. If the conductivity increases too much, so that the voltage decreases significantly, some of the electrodes which are switched out to decrease the area thus provide a higher voltage for the same current.
In a preferred form, a batch process is used in which contaminated water is pumped into a reactor. The electrodes are activated, causing some of the material to go into the water to form a coagulating agent which attaches itself to the pollutant particles (molecules, ions etc). Simultaneously, liberated gas bubbles capture the coagulated pollutants and float them to the surface from where the pollutants are easily removed.
A power supply and electrode combination can be used in which the sacrificial electrodes are worn away until they are almost entirely used up. The cost of sacrificial electrodes equates to USD 0.10 to USD 0.30 of metal per 1,000 litres of water treated, depending upon the degree of treatment required (that is, pollution encountered). Substantial cost savings compared with other water treatment methods are possible by almost completely consuming sacrificial electrodes.
A preferred embodiment is able to effectively treat water of varying conductivity, without needing to be specifically designed for treatment of a particular water type, or necessarily requiring regular monitoring and adjustment by an operator.
Water is pumped into a tank, the electrodes activated, the pollutant removed from the top and the processed water pumped out. The process repeats itself until such time as there is either no raw water available for processing or the processed water is receiving tank is full. The one system can handle a wide range of pollutants including most materials removed by particle filters, micro filtration and ultra filtration systems, ie, suspended solids down to and including large dissolved molecules, emulsified fats oils and greases (FOGs), heavy metal cations and phosphates among others. Suspended solids include bacteria, parasites, algae, paint and other pigments, carbon black, asbestos fibres, industrial grit, clay and similar.
The described system is able to treat water polluted with large dissolved molecules including humus, dye molecules, proteins, latex and polymers and similar. The system does not remove small non-polar molecules such as sugars, pesticides and similar. Nor does it remove low atomic number cations or anions (for example, it does not remove salinity). Removal rates are typically greater than 98%, with removal rates greater than 99.95% possible in some circumstances.